Plastics have become established within vehicle finishing as materials for vehicle components and also vehicle accessory components and components for installation in or on vehicles, both in the interior and in the exterior areas. Like other materials, plastics are coated or painted for decorative reasons (coloring, for example) and/or on account of technical suitability (light stability and weather resistance, for example) with corresponding coating compositions. One important requirement of a high-quality coating is its adhesion to the substrate surface. It is common knowledge that, particularly with the coating or painting of plastics, especially of apolar plastics, such as polypropylene (PP) in pure form or in modified form (through addition, for example, of ethylene-propylene-diene copolymers (EPDM)), problems of adhesion, in some cases serious, to the plastic substrate may occur. In order to achieve acceptable adhesion of the coating composition in question, apolar plastics of these kinds are conventionally subjected to a surface-activating pretreatment. The most frequently employed methods are flaming, plasma treatment, and corona discharge. Likewise known for the purpose of improving adhesion is the use of adhesion-promoting substances, especially chlorinated polyolefins. The adhesion-promoting substances are employed, for example, by way of adhesion primers, which comprise the adhesion-promoting substances and are applied to the plastic substrate in a separate coating operation. Likewise possible is the direct addition of adhesion-promoting substances to the coating composition with which the decorative and/or technically suitable coating is to be produced.
Even in the case of surface-activating pretreatment, the adhesion of coating compositions to plastic substrates is not always sufficient, and so, for example, weathering effects or mechanical stress cause successive delamination of coatings from painted plastics components. Nor does the use of adhesion-promoting substances lead to optimum adhesion properties in every case. A further factor is that the use of adhesion-promoting substances, especially chlorinated polyolefins, is very disadvantageous from an environmental standpoint. When aqueous coating compositions are used, which are becoming increasingly established for environmental reasons in the coating of plastics as well, the adhesion problems are exacerbated, especially in the painting of apolar plastic substrates, owing to the differences in polarity between the two media: the plastic substrate and the coating composition.
Patent application DE 199 61 983 A1 discloses a method for coating plastics, especially apolar plastics, with water-based, adhesion-promoting coating materials, the coatings obtained by the method exhibiting effective adhesion. Prior to the application of the coating materials, however, the plastic substrate must be cleaned with at least one specific organic solvent. Prior to the cleaning, there may be a heat-treatment operation on the substrate, with the temperature and time conditions stated for the operation being situated in the range from 50 to 80° C. for about 30 to 90 minutes. Cleaning is generally followed by an operation to evaporate the organic solvents used for cleaning, in the time range of one or more hours, before the coating material is applied. The coating material, moreover, comprises at least one adhesion promoter, especially a chlorinated polyolefin.